In standard fashion, a low-height (or low-altitude) flight phase, called an LLF (for “Low Level Flight”) phase, allows an aircraft to fly at low height, especially so as to follow as closely as possible the terrain overflown, in particular to avoid being pinpointed, while eliminating any risk of collision with a part of said terrain. Such an LLF phase is generally situated at a predetermined terrain height, for example at 500 feet (about 150 meters).
In particular, for a military aircraft, especially a military transport airplane, one of the objectives of a low-height flight is to exploit the masking in relation to the terrain in order to protect itself from threats in hostile geographical regions. Accordingly, in particular to carry out instrument flight operations of IMC (“Instrument Meteorological Conditions”) type, a three-dimensional reference trajectory is computed in standard fashion (taking into account the terrain overflown), and the aircraft is guided along this reference trajectory (either automatically with the aid of an automatic piloting system, or manually by following indications presented by a flight director).
The terrain used for the computation of the reference trajectory is the terrain situated in a corridor, termed the safety corridor hereinafter, around the horizontal part of the reference trajectory.
In standard fashion, the width of the safety corridor is predefined to ensure flight safety in flight of IMC type, by considering all the flight conditions which could cause the aircraft to deviate from its reference trajectory.
The deviations with respect to the reference trajectory can be caused by faults with systems of the aircraft. Generally, the safety corridor is symmetric on either side of the horizontal trajectory, deviations to the right or to the left of the trajectory being equiprobable. The width of the safety corridor is the result of the sum of various error values which correspond to the performance of the various systems which contribute to the computation and to the flight of the trajectory. This entails the various causes capable of generating lateral deviations with respect to the reference trajectory. By way of example, it is possible to cite a guidance-related error value which is determined by measuring the maximum deviation by considering all the flight conditions as well as the faults affecting a guidance function, and an error value relating to the determination of the aircraft's position, which is used to implement the guidance function. The guidance function (implementation via an automatic piloting system or a flight director) relies, in fact, on a very precise and very reliable item of information on the position of the aircraft.
When a fault with one of the systems necessary for the proper progress of an LLF operation is detected, the crew is alerted and must trigger an escape maneuver, since continuation of the flight under these degraded conditions is no longer safe. This escape maneuver consists in making the aircraft climb to a safety altitude at which the aircraft no longer risks colliding with the terrain overflown. In standard fashion, during the escape maneuver, the aircraft remains guided laterally along the lateral (or horizontal) part of the reference trajectory and a maximum climb rate is applied to it. A part of the safety corridor also serves to cover lateral deviations of the aircraft with respect to the reference trajectory during an escape maneuver (subsequent for example to a fault with a system).
In particular, the escape maneuver can be initiated subsequent to a fault affecting one of the position sensors which make it possible to calculate the position of the aircraft. The minimum number of position sensors installed aboard the aircraft depends on the safety level envisaged. To guide an aircraft in complete safety along a trajectory situated in a corridor, in flight of IMC type, at least two independent sensors of position are necessary. As in the aforementioned current procedure, the aircraft must also be guided along the horizontal part of the trajectory during the escape maneuver; two position sensors are also necessary during this maneuver. Consequently, to permit LLF operations in flight of IMC type, at least three independent sensors of position must be provided (since two sensors are necessary for guidance during the escape maneuver, and since this maneuver may be triggered subsequent to a fault with one of these sensors).